Electrical segregation circuit



Oct. 31, 1939. R. J. KEMP El AL 2,177,723

ELECTRICAL SEGREGATION CIRCUIT Filed April 1-6, 1937 INVENTOR; ROLAND J-KEM DA V/D J. FEW/MES BY Z ATTORNEY Patented Oct. 31, 1939 UNITED STATESELECTRICAL SEGREGATION CIRCUIT Roland John Kemp and David John Fewings,

Chelmsford, England, assignors to Radio Corporation of America, acorporation of Delaware Application April 16, 1937, Serial No. 137,166In Great Britain April 29, 1936 6 Claims.

This invention relates to television and like receivers and moreparticularly to synchronizing signal apparatus suitable for use incathode ray tube television and like receivers.

It is usual in modern television and like practice to transmit squaretopped line synchronizing signals at the end of each scanning line andsquare topped framing signals at the end of each frame these linesynchronizing and frame synchronizing pulses being employed at thereceiver to synchronize scanning action thereat. As a general rule allthe synchronizing signals are blacker than black.that is to say they areof greater amplitude than the picture signal amplitude corresponding toblack in the subject of transmission-and the framing pulses are ofsubstantially longer duration than the synchronizing pulses each framingpulse generally extending over several line periods. In some cases theframing pulses are of greater amplitude than the line pulses. It is,however, by no means a simple matter (owing largely to loss ofefliciency at the transmitter) to make the difference in amplitudebetween synchronizing pulses and framing pulses as received great enoughto be relied upon for amplitude selection and,

moreover, there are obvious practical advantages in transmittingsynchronizing signals and framing pulses at the same amplitude andmaking the difference between the two varieties of pulse one ofdifferent duration only.

In modern cathode ray tube television and like receivers intended toco-operate with transmitters of the kind wherein framing signals ofsubstantially longer duration than synchronizing signals are transmittedover a common channel,

it is usual practice to rely upon frequency selective networks i. e.networks relying upon resonant circuits, and the like, for selecting theone type of synchronizing signal from the other but to in general suchfrequency selective networks are not very reliable nor effective. Theyare, moreover, not as simple as is desirable.

The object of the present invention is to provide improved andsimplified apparatus for selecting framing signals from linesynchronizing signals in a television or like receiver for cooperatingwith a transmitter of the kind transmitting two varieties ofsynchronizing signals of different lengths. As will be seen later ap- 50paratus in accordance with this invention does not involve the use oftuned circuits or like frequency selective means, but, neverthelessreliable selective action is provided.

According to the main feature of this inven- 55 tion received linesynchronizing signals and framing signals appearing in a common channeland distinguished from one another at least in that the two varieties ofsignals are of different duration or different amplitude, or both, 60are separated by means including a selective device or circuit having anoperating input voltage-output current curve of substantially differentslope at different parts thereof. Where the line synchronizing andframing signals as received are of the same amplitude they are first 5applied to a time circuit network to derive therefrom differentamplitude signals which are then applied to the selective device ornetwork. Where, however, the synchronizing and framing signals are ofdifferent amplitudes they may be applied direct to the selective deviceor network. Preferably the selective device or network is such as totransform different amplitude pulses into different polarity pulses.

The invention is illustrated in and further explained in connection withthe accompanying drawing, in which Fig. 1 is an embodiment of ourinvention,

Figs. 2 and 3 are explanatory curves,

Fig. 4 is a modified arrangement,

Fig. 5 is an explanatory curve,

Fig. 6 is an application of our invention,

Figs. '7, 8, and 9 are explanatory curves,

Fig. 10 is an alternative arrangement, and

Fig. 11 is a further modified arrangement.

Referring to Fig. 1 which shows one way of carrying out this inventionas applied to a system wherein the line synchronizing and framingsignals as received are of the same amplitude but different durations,the mixed synchronizing signals are applied through a condenser l acrossa time constant circuit which may, for example, consist of a shuntresistance-condenser combination 2, 3, and which is inserted in thecathode lead of a screen grid valve 4 whose anode 5 is connected througha resistance 6 to a point I on an anode battery or other source 8, thescreen grid 9 being connected through a resistance Ill to a point IIwhich is positive with respect to the point I. The time constant circuit2, 3, is so designed as to accept the relatively long period framingpulses and heavily to attenuate the relatively short period linesynchronizing pulses. The control grid l2 and cathode l3 are tiedtogether. The operating potentials applied to the screen grid valve areso chosen that the operating anode current (ordinates)- anode voltage(abscissae) characteristic curve of the valve is of the well known formrepresented in Figure 2, exhibiting a rising portion followed by a dipfollowed by a second more steeply rising portion which ultimatelyflattens out in the well known way. The valve 4 is operated at asuitable point of this characteristic somewhere on the origin side ofthe dip e. g. at the point A which is at the crown of the first rise.Suppose a series of synchronizing signals of uniform amplitude butdifferent duration be applied across the circuit 2, 3. The action ofthis time constant circuit 2, 3, will be to transform the pulses ofsimilar amplitude but different duration into pulses of difierentamplitudes and short pulses (line synchronizing signals) beingtransformed into small amplitude pulses such as are represented at-L inFig. 2 while the long pulses (framing signals) will be transformed intolarger amplitude pulses such as are represented at F in Fig. 2. Thesmall amplitude pulses L (due to the line synchronizing signals) willcause a diminution of anode current in valve 4 (the valve swingoccurring over the negative sloping portion BC of the dip) whereas thelarge amplitude pulses F will cause a rise in anode current since thevalve swing carries over to a point such as B near the top of the secondrising portion in the characteristic curve. As a result the line pulsesappear in the anode circuit of the valve as of one polarity and theframing pulses appear of the opposite polarity. These pulses, which canbe taken off through a condenser H from the anode circuit, can,therefore, very easily be separated, e. g. by suitable rectifiercircuits (not shown). For example, the anode pulses may be applieddirectly or through a phase reversing valve to a rectifier circuit andobviously the rectified resultant will consist either of the line pulsesonly or of the framing pulses only according as to whether the energychannel to the rectifier includes a phase reversal valve or not.

If desired, the above described arrangement may be modified by derivingthe one variety of pulse from the anode (through condenser I4) and theother from the screen grid 9 (for example through condenser l5) by meansof suitable rectifier circuits energized from these two electrodes, forobviously the pulses from the screen grid will be in reversed phase ascompared to those from the anode.

Preferably the screen grid valve and circuit is such that the operatingcharacteristic is as shown in Fig. 3 rather than as shown in Fig. 2, i.e. such that the dip in the anode currentanode voltage curve extendsbelow the abscissa line by an amount approximately equal to the amountby which the first rise in the curve extends above the abscissa line.This will obviously result in improved sensitivity. If desired, where avalve arrangement exhibiting the form of characteristic shown in Fig. 3is employed, it may be operated over that portion DEF of the curve lyingbetween the origin D and the first point of crossing F of the abscissaline, line pulses causing a swing over the positively sloping part DE ofthis portion and framing pulses carrying the swing over the negativelysloping part EF. This method of operation is, however, not preferred, itbeing preferred to operate in the manner already described withreference to Fig. 2.

In the modification diagrammatically represented in Fig. 4 the mixedsynchronizing signals of different duration are applied through a timeconstant circuit within the broken line rectangle TC to the screen grid9 of a screen grid valve 4 which is so operated as to exhibit an anodecurrent (ordinates)-screen grid voltage (abscissae) characteristic asshown in Fig. 5 said characteristic rising to a maximum and then fallingaway. For this method of operation the operating point is chosen at apoint such as G on the positively sloping part HGK of the curve so thatthe small amplitude signals such as L (due to short duration linesignals) result in a swing up to or near the crown of the curve but thelarger amplitude signals such as F (due to long, framing signals) causea swing well past this point X and down to a point such as J near theabscissa line. As before the control grid l2 and cathode' l3 are tiedtogether and the time circuit again attenuates the short pulses just asin Fig. 1, being, however, now inserted in the signal supply lead tothescreen grid.

Where the line and framing pulses as received are already ofsufliciently different amplitudes the above described and illustratedembodiments may be modified by omission of the time constant circuit.

It is important to note that all the above arrangements transformdifferent amplitude signals of the same polarity into signals ofopposite polarity, thus making separation exceedingly simple.

In the arrangement shown in Fig. 6 the mixed synchronizing signals,which are arranged to have their peaks in the positive direction, areapplied through a condenser I and regulating potentiometer I8 betweencontrol grid I6 and cathode ll of a triode I9 whose anode cathode space20l1 is shunted by a condenser 2|. The anode 20 of the triode receivespositive potential from a tap 22 on a potentiometer resistance 23shunted across a potential source (not shown) through a circuitincluding a resistance 6 and the anode-cathode space 5'--|3' of a screengrid valve 4' whose control grid I2 and cathode I3 are connectedtogether and to the anode 20 of the triode l9. Positive potentialexceeding that on the anode 5 is applied from tap 24 via resistance H)to the screen grid 9'. In the absence of applied signals the condenser2| charges to a steady potential which depends inter alia on therelative D. C. resistances of the two series connected valves I9, 4'.Each successive positive peak of the applied signals reduces for aninstant the resistance of the triode I9 and.the condenser 2| dischargesslightly through this valve. Obviously a long duration (framing) pulsewill cause a greater discharge than a short duration (line) pulse.Between signal peaks the condenser 2l charges up through the screen gridvalve and the circuit parameters are so selected that this recharging isreasonably rapid. Thus the voltage at the anode 20 will vary after themanner shown in Fig. 7 the shallow narrow dips L being due to shortsignals (line signals) and the deep wide dip F being due tolong signals(frame signals). The voltage across the screen grid valve 4' willclearly vary in an opposite manner, i. e. it will be increasedmomentarily and slightly for each line pulse and will be increased muchmore and for a longer period for each framing pulse. The operatingpotentials on the screen grid valve are so adjusted that it exhibits ananode current (ordinates) anode voltage (abscissae) characteristic curveas shown in Fig. 8 with a rising portion MN followed by a shallow,fairly flat bottomed dip NQ followed by a more steeply rising portion QPwhich ultimately flattens oil. The operating point is chosen at a pointsuch as 0 on the flat bottom of the dip so that the small increases ofanode voltage due to line pulses produce little or no change in anodecurrent (the valve swing remains on the fairly flat bottom NQ of thedip) whereas the large increases in anode voltage due to framing pulsesswing the valve to or near the top of the second rise Q? in the curvethus causing large anode current changes. Accordingly framing pulses maybe taken via con- &

denser l4 from the anode and due to the flattening which follows thesteep rise QP of the curve, these pulses will be fairly fiat topped asis required for good synchronizing action. Since the condenser 2|recharges rapidly, the return to the normal operating point is almostimmediate. The pulses-obtained from the anode will be negative indirection, but if positive pulses are required they may obviously beobtained (e. g. through a condenser l5) from the screen grid 9'.

The screen grid valve in the last described embodiment may be replacedby any other suitable charging device exhibiting an applied voltage(abscissae) output-current characteristic curve part of which consists(as shown in Fig. 9) v of a fairly flat portion followed by a risingportion which (preferably) flattens oif sharply at the top. For example,a suitably operated diode might replace the screen grid valve.

Again the screen grid valve might be replaced by a neon or otherelectric discharge lamp 25 as shown in Fig. 10. In this case the trlodel9 must be biased practically to out off and the arrangement made suchthat on switching on the anode.

potential the lamp will flash and permit the condenser to charge to avalue such that the lamp is extinguished. The signals are applied as inFig. 6. Each successive signal peak will discharge the condenser 2|slightly so that the voltage across the lamp rises. The parameters ofthe circuit are so chosen that when a framing pulse arrives the morecomplete discharging which then occurs allows the potential across thelamp 25 to rise above the ignition potential so that the lamp flashes,recharging the condenser 2!. The cycle is then repeated. Framing pulsescan therefore be taken through condenser l4" from that terminal of thelamp 25 remote from the triode In a further modification illustrated inFig. 11 the triode l9 of Fig. 6 is replaced by a high resistance 26which is shunted by the condenser 2|. In this case the signals areapplied through condenser in the negative sense across the resistance 26and must be of considerable magnitude-e. g. they may be amplified beforeapplication. The operation is much the same as in Fig. 6 except that thecondenser M is discharged directlyby negative pulses and not, as before,indirectly by positive pulses which vary the resistance of a valve (IQof Fig. 6) shunting the condenser.

of like amplitude and differing? duration are transformed into signalsof difiering amplitudes and differing polarities.

2. A. circuit arrangement for separating television signals which are ofdifierent time duration comprising a selective device having anoperating input voltage=output current curve of sub- (ordinates) signalsonto .said thermionic device, means forbiasing' said thermionic deviceto operate on a portion of its characteristic curve at a point on thepositively sloping portion thereof so that input voltages of largeamplitudes cause a swing over the crown of the curve into thenegativelysloping portion thereof.

3. A. circuit arrangement for separating television synchronizing orlike signals which are of different time duration, said arrangementcomprising means for transforming the signals of different time durationinto signals of different amplitudes, a selective device comprising athermi onic valve having an operating input voltageoutput current curveof substantially different slope at different parts and means forapplying the transformed, amplitude-differentiated signals as inputsignals to said valve means for biasing said valve whereby signals ofone amplitude sweep over a portion of its characteristic ofsubstantially different slope from that swept over as a result of theapplication of signals of the other amplitude, the valve being sooperated that the dip in the anode current-anode voltage curve extendsbelow the abscissae line by an amount approximately equal to the amountby which the first rise in. the curve extends above the abscissae line.

4. An arrangement as claimed in claim 3, wherein signals of differentduration which are to be separated are applied across a time constantcircuit which attenuates the signals. of shorter duration relatively tothose of longer duration, said circuit being connected in the cathodelead of the valve, said valve being of the screen grid type.

5. An arrangement as claimed in claim 3 wherein said selective valve isof the screen grid type, and a time constant circuit connected in thescreen grid circuit thereof, said time constant circuit serving toattenuate the signals of shorter duration relatively to those of longerduration.

6. A circuit arrangement for separating television synchronizingsignals, said arrangement comprising a selective device including avalve having anode, cathode and at least one control electrode, saidvalve having an operating input voltage, output current curve ofsubstantially different slope at different parts of the curve, a diodeso operated that its anode current-anode voltage characteristic includesa fairly flat part followed by a steeply rising part, the operatingpoint being chosen on the fairly flat part so that small increases ofanode voltage produce little change in anode current whereas largeincreases of anode voltage cause substantial changes in anode current,said diode being connected in a. current carrying electrode circuit ofthe thermionic valve, and means for applying the signals to be separatedas input signals to said thermionic valve, the latter being so operatedthat for signals of one amplitude it sweeps over a portion of itscharacteristic of substantially different slope from that swept over asa result of the application of signals of the other amplitude.

ROLAND JOHN KEMP. DAVID JOHN FEW'INGS.

